This photo shows only a portion of the chocolate used as initial estimates were slightly low. We also bought too many packs of biscuits and only ended up using 5.

STEP ONE
Make a mould 500cm across and 10cm deep from cardboard, covering it with cling film or foil (or both if you run out like I did) to stop the chocolate sticking to It. Like me you may need to buy and drink and whole crate of Lager to get some cardboard, it was torture :-)

STEP TWO
Melt up 4kg of chocolate, this is enough to provide a decently strong base for this mighty biscuit. Pour this into the base and put in the fridge to set. Your half meter mould should just fit in a standard sized fridge - If not it might take a while to set.

STEP THREE
Whilst your base of chocolate sets get to work bashing the life out of those biscuits. Crush 5 packets to a nice consistency.

STEP FOUR
Melt half your tub of butter in the microwave and mix the delicious molten butter into half of your crushed biscuits - stir fast or the biscuit goes soggy!

STEP FIVE
Remove your base from the fridge and, after pausing for 5 minutes to admire its spectacular size, spread your buttered biscuit mix over leaving a half inch gap around the outside.

STEP SIX
Melt up half your marshmallows in the microwave and pour over the biscuit. I used a fork to push the quickly setting marshmallow around to an even coat which seals the side of the biscuit.